Many areas have been identified where it is desirable to have an artificial sense of touch. The most commonly mentioned applications include the field of automation, particularly industrial robotry, tele-operation as employed in remote manipulation underwater, in spacecraft, or with extraterrestrial landing vehicles, and also in the field of prosthetic devices for amputees or those handicapped with loss of nerve function. In all of these applications, information about an object being contacted or manipulated is necessary to make a characterization of the object such as size, shape, weight, orientation and other contact aspects, and then based on such characterization computer-controlled decisions can be made such as properly grasping, moving, manipulating, placing and releasing the object.
Considering the area of industrial robotry, which appears to be a particularly good candidate for devices having tactile sensing capability, it is estimated that approximately 75% of the manufacturing conducted in the United States is low volume or batch operations performed by manual labor and accounting for approximately 30% of the gross national product. Machine replacement of human labor in at least a portion of such activities can result in improved accuracy, quality, speed, safety, efficiency and economy. It has been estimated that the cost of human labor in only the last decade has increased at a rate of more than five times the cost of robotic labor. In addition, this estimate does not take into consideration that robots can operate machine tools, for example, with a repeatable precision that may take a human worker many years to acquire. Manufacturing operations which potentially could be accomplished by devices having tactile sensing capabilities include assembly, fast adaptive grasping or the pick-up of randomly oriented parts from assembly lines, bin picking, grinding, deburring, polishing and welding among others.
Tactile sensing systems have been introduced in recent years but generally have met with little success and are of limited practical utility. For example, typical prior art tactile sensing systems are shown in U.S. Pat. No. 4,014,217 to Lagasse et al, and U.S. patent application Ser. No. 6-225,500 to Raibert et al entitled "Tactile Sensing System" and available from NTIS. The tactile pick-up system of Lagasse et al includes a continuous outer layer of material which has a variable electrical conductivity as a function of its state of compression. A matrix of measuring electrodes mounted to the layer of variable electric conductivity are operable to measure the electric field produced by compression of the layer in response to contact with an object. In this design, the continuous outer layer itself is electrically responsive to force exerted by contact with an object. When compressed, the electrical conductivity of the layer is altered at the adjacent to the point of contact and this changed condition is detected by the electrodes.
One problem of the Lagasse et al design is low contrast caused by the continuous nature of the pressure-sensing electrically conductive outer layer. While placement of an object on the outer layer will cause it to deflect and thus change the electrical conductivity of the layer immediately beneath the object, such deflection cannot be confined or isolated to the limited area of the object's dimensions. This is true because portions of the outer layer immediately adjacent those portions contacting the object must also be deflected or compressed at least to some degree by the object. The compression of adjoining or contiguous portions of the layer causes a current to be conducted there which is sensed by the electrodes. Therefore, instead of sensing current only at those locations along the outer layer where an object actually makes contact, prior art devices such as Lagasse et al tend to produce false or incorrect signals from adjacent areas resulting in relatively low resolution or inaccurate characterization of an object's presence, pressure, size, shape and similar contact parameters. To the extent that such false signals or "cross-talk" may be circumvented in Lagasse et al, complex pattern recognition systems would be required.
For purposes of discussing the prior art and also the structure and advantages of this invention, the term contrast will be used in reference to the extent which a tactile sensing system is capable of sensing the boundaries of an object contacting the sensitive surface of the device. The term resolution will refer to the minimum change in load that can be detected, both in terms of the magnitude and location of such load on the sensing surface. Therefore, in describing a tactile sensing system having high or low contrast, for example, such terms may be considered as an indication of the system's ability or inability to accurately characterize the boundaries of an object contacting the sensing surface of the device.
In the Raibert et al patent application, a tactile sensing system is disclosed which is similar to the Lagasse et al system in that a layer or sheet of material is disposed over an array or matrix of measuring electrodes which are operable to produce a signal in response to contact of an object with the surface of the layer. A sheet of pressure sensitive conductive plastic forms the layer in Raibert and the resistivity of the sheet changes in a known manner as a function of its deformation. The array or matrix of measuring electrodes senses such change in resistivity and relays that information to electrical components for processing. For the same reasons as discussed in connection with the Lagasse et al patent, this approach suffers from a limited resolution capability since it is difficult to localize or isolate the strain imposed by contact with an object to be sensed to a limited area where a continuous sensing surface is utilized.
Additionally, a common limitation of the overlayer material used in the above cited prior art is that such material is not rugged in construction and may be susceptible to failure particularly in the harsh environment found in many manufacturing operations. A cut, abrasion or other surface irregularity could easily alter the conductive or resistive properties of the outer layers in Lagasse et al and Raibert respectively. Since the sensing capability of each system is dependent on the integrity of the outer layer, which layer accomplishes the actual sensing function, it is axiomatic that change to the layer would render the entire system inoperable. Moreover, electrically conductive or resistive elastomers or other polymers generally exhibit poor hysterisis, excessive set and poor mechanical strength. All of these physical properties make such materials undesirable for use in many tactile sensing applications.
It has therefore been an object of this invention to provide a tactile sensing system which is capable of high contrast sensing of the magnitude and location of the deflection caused by contact of an object to be manipulated with the sensing surface of the system.
It is another object herein to provide a tactile sensing system capable of sensing parameters when contacting an object such as presence, pressure, size, shape, location and orientation among others.
It is a further object to provide a tactile sensing system having the dual capability of sensing shear and normal loads imposed by contact with an object.
Another object herein is to provide a tactile sensing system having the combination of high contrast normal force detecting capability and separate moderate resolution shear force detecting capability.
It is still another object to provide a tactile sensing system having a touch surface with individual sensing means for contacting an object which are separate from one another and exhibit little or no deflection when adjacent sensing means contact an object.
It is a still further object herein to provide a touch surface of rugged construction which acts to transfer load in the form of mechanical movement for measurement by sensing means.
It is another object of the invention to provide a sensing system having a touch surface with sensitive sites whose compliance may be controlled through material choice and/or geometrical configuration for varying the response of such sites to deflection by contact with an object.